DESCRIPTION

sem_open()
creates a new POSIX semaphore or opens an existing semaphore.
The semaphore is identified by
name.
For details of the construction of
name,
see
sem_overview(7).

The
oflag
argument specifies flags that control the operation of the call.
(Definitions of the flags values can be obtained by including
<fcntl.h>.)
If
O_CREAT
is specified in
oflag,
then the semaphore is created if
it does not already exist.
The owner (user ID) of the semaphore is set to the effective
user ID of the calling process.
The group ownership (group ID) is set to the effective group ID
of the calling process.
If both
O_CREAT
and
O_EXCL
are specified in
oflag,
then an error is returned if a semaphore with the given
name
already exists.

If
O_CREAT
is specified in
oflag,
then two additional arguments must be supplied.
The
mode
argument specifies the permissions to be placed on the new semaphore,
as for
open(2).
(Symbolic definitions for the permissions bits can be obtained by including
<sys/stat.h>.)
The permissions settings are masked against the process umask.
Both read and write permission should be granted to each class of
user that will access the semaphore.
The
value
argument specifies the initial value for the new semaphore.
If
O_CREAT
is specified, and a semaphore with the given
name
already exists, then
mode
and
value
are ignored.

RETURN VALUE

On success,
sem_open()
returns the address of the new semaphore;
this address is used when calling other semaphore-related functions.
On error,
sem_open()
returns
SEM_FAILED,
with
errno
set to indicate the error.

ERRORS

EACCES

The semaphore exists, but the caller does not have permission to
open it.

EEXIST

Both
O_CREAT
and
O_EXCL
were specified in
oflag,
but a semaphore with this
name
already exists.

EINVAL

value
was greater than
SEM_VALUE_MAX.

EINVAL

name
consists of just "/", followed by no other characters.

EMFILE

The process already has the maximum number of files and open.

ENAMETOOLONG

name
was too long.

ENFILE

The system limit on the total number of open files has been reached.

ENOENT

The
O_CREAT
flag was not specified in
oflag
and no semaphore with this
name
exists;
or,
O_CREAT
was specified, but
name
wasn't well formed.